The copies of the genes for S25 and S17 rRNA in the Tetrahymena macronucleus have been shown to be extrachromosomal and are amplified from one or a few copies contained in the germinal micronucleus. The timing and mechanism by which this amplification occurs will be studied during conjugation. A starvation-refeeding regimen exists after which the only nuclear DNA synthesized is rDNA. Using this method to selectively label rDNA and the proteins synthesized concomitantly with rDNA, studies are being carried out on the subunit structure and on the histones of rChromatin during different states of activity of the genes for rRNA. Since only one or a few copies of the gene for rRNA in Tetrahymena is genetically transmitted, while 300-400 genes for S5 RNA are contained in the germinal nucleus, studies are under way to isolate and purify the S5 genes to compare their structure and homogeneity with S5 genes of other organisms and with the rDNA of Tetrahymena. BIBLIOGRAPHIC REFERENCES: Kimmel, A.R. and M.A. Gorovsky. Number of S5 and tRNA genes in macro- and micro-nuclei of Tetrahymena pyriformis. Chromosoma. Accepted for publication, 11/17. Mathis, D.J. and M.A. Gorovsky. Subunit structure of rDNA-containing chromatin. Biochemistry. Accepted for publication, 11/75.